This invention relates generally to radios and more particularly to squelch control for AM radios.
In amplitude modulated (AM) radios, the amplitude of the carrier wave is made to vary corresponding to the fluctuations of a sound wave, a television image, or other information which is to be conveyed. AM radio transmission is the oldest and simplest form of modulation and is used almost exclusively for aviation settings and is consistently used in directional assistance for marine applications.
A major problem with AM radio transmissions is the removal of noise from the transmission and also identifying when a signal should be squelched. Squelching of a signal is done when the signal-to-noise ratio exceeds an acceptable level making the transmission unintelligible.
The signal-to-noise (SNR) problem has been compounded through the use of much narrower bandwidth. In some applications, the bandwidth has been cut in thirds; thereby lowering the SNR which acceptable.
In today's narrow band AM voice communication system for aviation, classical noise squelch circuits do not work when the channel spacing is reduced to 8.33 KHz.
Some of the traditional methods of controlling squelch are:
Audio Amplification/Rectification: The detected audio is amplified and rectified. The rectified audio is used to control the squelch gate. This method does not work reliably in a receiver that is under Automatic Gain Control (AGC) control when there is no signal present (noise only). It also does not work when there is only a carrier present.
RF Amplification/Rectification: The rectified carrier voltage is used to control the squelch gate. This method does not work for narrow band AM because the narrow band noise rectifies to almost the same level as a carrier.
Sub-Audible Tone: In this technique, a sub-audible tone is transmitted with the carrier and audio. The tone is detected in the receiver and used to control the squelch gate. This does not work in a system where there is no tone being transmitted and as such is not applicable to many AM systems.
Noise Amplified/Rectified: The rectified noise is used to control the squelch gate. This does not work in narrow band AM systems because there is no excess noise power above the audio frequency range.
Monitoring AGC Voltage: This technique monitors the AGC voltage and uses it to control the squelch gate. This is not a desirable method for many applications because the threshold does not remain constant over temperature, and because there is no way of distinguishing between noise changing the AGC voltage and signal changing the AGC voltage (at very low RF levels).
Without an effective squelching mechanism, the effectiveness of the radio is jeopardized.
It is clear that there is a significant and increasing need for an improved squelch control for AM radios as their bandwidths are narrowed.